N-Heterocyclic Carbene Catalyzed Domino Reactions of Formylcyclopropane 1,1-Diesters: Construction of a 6-5-5 Tricyclic Pyrrolo[1,2-a]indole

Recent Advances in NHC-Catalyzed Chemoselective Activation of Carbonyl Compounds

N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.

NHC-Catalyzed Chemoselective Reactions of Enals and Cyclopropylcarbaldehydes for Access to Chiral Dihydropyranone Derivatives

An N-heterocyclic carbene (NHC)-catalyzed chemoselective activation reaction of 1-cyclopropylcarbaldehydes and α-alkynyl enals is reported. NHC selectively catalyzes 1-cyclopropylaldehydes, followed by a [2 + 4] cycloaddition reaction with α-alkynyl enals. The target dihydropyranone derivatives bearing different substituents and substitution patterns can be obtained in good to excellent yields with excellent enantio- and diastereoselectivities under mild conditions.

Electron Donor-Acceptor Complex Induced Fused Indoles with Hypervalent Iodine(III) Reagents

An operationally simple and efficient method for the cyclization of tertiary amines and hypervalent iodine reagents enabled by an EDA complex has been developed. A series of [1,2-α]indoles derivatives were obtained in good yields, including some key intermediates for the synthesis of biologically active molecules. In addition, this established strategy features a broad substrate scope and good functional group tolerance.

Metal-free polychloromethyl radical-initiated cyclization of unactivated N-allylindoles towards pyrrolo[1,2-a]indoles

A metal-free polychloromethyl radical-initiated cyclization of unactivated alkenes was developed using CH₂Cl₂ and CHCl₃ as the di- and trichloromethyl radical sources. Variously substituted N-allyl-indoles were successfully transformed into the corresponding C2-(di- and trichloromethyl) pyrrolo[1,2-a]indoles in moderate to good yields. This reaction has a broad substrate scope and good functional group tolerance. Dibromomethylated products can also be obtained using CH₂Br₂ under standard conditions.

Umpolung of donor–acceptor cyclopropanes via N-heterocyclic carbene organic catalysis

A carbene-catalyzed formal umpolung of donor–acceptor (D–A) cyclopropanes is disclosed with chiral spirocyclic lactones bearing multiple functional groups afforded with excellent enantio- and diastereoselectivities.

Predicting the origin of selectivity in NHC-catalyzed ring opening of formylcyclopropane: a theoretical investigation

Using density functional theory, we investigated the origin of selectivity in the N-heterocyclic carbene (NHC)-catalyzed transformation of formylcyclopropane with an alkylidene oxindole.

Application of Cp2TiCl-Promoted Radical-Induced Cyclization: An Expeditious Access to [a]-Annelated Indoles

An efficient and novel route for assembling pyrrolo/piperido[1,2-a]indoles is portrayed involving a radical-mediated reductive epoxide opening reaction of N-tethered epoxy-indoles that trigger facile intramolecular cyclization followed by an oxidative quenching step. Capitalizing on the operational simplicity of the method involving just two steps and use of an efficient C-C bond-forming reaction, this radical-based protocol enables the modular assembly of an important class of N-fused indole derivatives with versatile functional and structural diversity.

Asymmetric N-alkylation of indoles with isatins catalyzed by N-heterocyclic carbene: efficient synthesis of functionalized cyclic N,O-aminal indole derivatives

Enantioselective N-alkylation of indole-2-formaldehydes with isatins catalyzed by NHCs is described for efficient synthesis of functionalized cyclic N,O-aminal indole derivatives.

A tandem dearomatization/rearomatization strategy: enantioselective N-heterocyclic carbene-catalyzed α-arylation

In this study, the first example of carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described.

In this study, the first example of the carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described. This protocol enables the efficient assembly of chiral dihydrocinnolinone derivatives in good yields with excellent enantioselectivities (up to 99% ee). Moreover, this strategy enables not only the highly enantioselective NHC-catalyzed nucleophilic aromatic substitution, but also a formal Csp²–Csp³ bond formation.

Bioinspired total synthesis and structural revision of yuremamine, an alkaloid from the entheogenic plant Mimosa tenuiflora

Guided by a biosynthetic hypothesis, a serendipitous total synthesis of yuremamine has resulted in its structural revision from the putative pyrroloindole (1) to the flavonoidal indole (2), which was initially proposed as a biosynthetic intermediate.

NHC-Catalyzed Asymmetric Synthesis of Functionalized Succinimides from Enals and α-Ketoamides

The efficient asymmetric synthesis of highly substituted succinimides from α,β-unsaturated aldehydes and α-ketoamides via NHC-catalyzed [3+2] cycloaddition has been developed. The new scalable protocol significantly expands the utility of NHC catalysis for the synthesis of heterocycles and provides easy access to assemble a wide range of succinimides from simple starting materials.

N-heterocyclic carbene-catalyzed Ireland-Coates Claisen rearrangement: synthesis of functionalized β-lactones

The N-heterocyclic carbene (NHC)-catalyzed Claisen rearrangement of hybrid Ireland-Coates structures has been achieved, allowing the stereoselective synthesis of highly functionalized β-lactones. The reaction proceeds with high diastereoselectivity (>20:1) and affords a diverse range of β-lactone fused cyclopentanes. Mechanistic studies are detailed.

N-heterocyclic carbene-catalyzed cascade annulation reaction of o-vinylarylaldehydes with nitrosoarenes: one-step assembly of functionalized 2,3-benzoxazin-4-ones

The NHC-catalyzed reactions of ortho electron-deficient vinyl substituted arylaldehydes with nitrosoarenes were studied. The reactions produced multifunctional 2,3-benzoxazin-4-ones in good to excellent yields via a cascade aza-benzoin reaction between aldehyde and nitroso groups followed by an intramolecular oxo-Michael addition. The resulting 1-acetate substituted 2,3-benzoxazinones were transformed into a new type of β-hydroxycarboxylate derivatives or 3-oxo-1-isobenzofuranacetates, respectively, under different reductive conditions. This work not only provides a simple and efficient method for the construction of multifunctional 2,3-benzoxazin-4-ones of potential pharmacological interest, but also expands the application of NHC-catalyzed cascade reactions in the formation of carbon-heteroatom and heteroatom-heteroatom bonds.